CANCUN, Mexico— Leaders from 194 countries gathering to discuss climate change in Cancun this week face a mounting body of evidence of the depth of this global crisis and a rapidly closing window of time to avoid its most catastrophic effects.

The headlines of 2010 have been full of climate-related disasters: Heat waves and fires scorched Russia, flooding displaced millions in Pakistan, a 100-square-mile chunk of Greenland’s ice fell into the sea.

This year is on track to be the warmest on record, and that’s on top of a string of world temperature records set over the past 15 years. It’s an unmistakable signal that climate change is already underway and manmade greenhouse gases — pollution that builds up in the atmosphere and traps the sun’s heat near the Earth — are driving these changes.

Many of us had hoped the global leaders gathering in Copenhagen last year would finally take the steps necessary to begin dramatic reductions of carbon dioxide and other greenhouse pollutants and pull the planet back from the brink. Instead, Copenhagen was a disappointing display of timidity, delay and business-as-usual politics in the face of the most sweeping problem ever to confront our species.

For those of us with the Center for Biological Diversity who are attending the talks in Cancun, our hopes are tempered with realism. Few expect it to be a watershed moment that turns the corner on this crisis; but that doesn’t mean we should allow world leaders to continue to forestall action.

In particular, it’s vital that the United States begin making significant cuts in greenhouse gas pollution. Since the United States is the largest historic emitter of greenhouse gas pollution, its lack of meaningful action is severely limiting the ability of the international community to move forward on a climate agreement.

Right now, atmospheric levels of CO2 are about 392 parts per million and likely to keep rising if left unchecked.

The consequences of allowing the Earth’s temperature to rise by even a few degrees are dire: 97 percent of the world’s coral reefs would disappear; billions more people would suffer from drought and limited drinking water; millions would be displaced by rising sea levels and cyclones; and agricultural yields would plummet.

Scientists have been telling us for years that atmospheric CO2 must be limited to no more than 350 parts per million to avoid the most disastrous consequences of global climate change. That number is key “if humanity wishes to preserve a planet similar to that one which civilization developed and to which life on Earth is adapted,” according to Dr. James Hansen, one of the world’s leading climatologists.

Hansen and his colleagues have charted a path to 350 ppm by the end of this century. We must reduce greenhouse gas pollution to 42 percent below 1990 levels by the year 2020, dramatically cut fossil fuel emissions, phase out coal-fired power plants, end large-scale deforestation and reforest cut-over areas.

To reach 350, global CO2 emissions must peak by 2015 and then begin a rapid decline. This won’t be easy, but it is doable, and the price of inaction will be devastating.

The steps taken so far simply don’t go far enough. Even viewed in the most optimistic light, the climate pledges made in Copenhagen would lead to atmospheric CO2 concentrations of 650 ppm, or a roughly 6-degree Fahrenheit increase in temperature by 2100. A temperature increase of this magnitude would result in widespread catastrophic impacts that far exceed what can rationally be considered safe. World leaders must address the deficiencies in the pledges reached at Copenhagen and provide a path to ensuring that CO2 concentrations are reduced to 350 ppm.

We remain hopeful that the end of 2010 will mark the beginning of a real and transformative shift toward addressing this global crisis. It can and must be done. Too many future generations of people, plants and animals are counting on us.